1. Field of the Invention
This invention relates to a driving circuit for an electric power semiconductor element such as a transistor, more particularly to a driving circuit for a semiconductor element capable of avoiding burnout due to reverse bias current.
2. Description of Related Art
FIG. 1 is a circuit diagram reported on p. 44 in "Application Manual for Inverter" published by "Denski Shoin" (Sept. 7, 1985).
In FIG. 1, reference numeral 2 designates a transistor being a semiconductor element which is driven, that is, to be controlled, and reference numberal 3 designates a driving circuit for the transistor.
The construction of the driving circuit 3 is as follows.
Reference numeral 15 designates a photocoupler which photoelectric-transfers an inputted signal from the exterior by converting it into a photo signal by a light emitting element 15a and receives an electric signal after the photo signal is converted into the electric signal by a light receiving element 15b. With an output terminal of this photocoupler 15, respective bases of an NPN transistor 13 being a first transistor and of a PNP transistor 14 being a second transistor are connected.
Respective collectors of the first and second transistors 13, 14 are connected with a positive electrode of a first power source 11 and a negative electrode of a second power source 12 being D.C. power sources connected in series respectively through respective resistances 17, 18.
Reference numeral 16 designaltes a resistance being connected between the base of the first transistor 13 and the first power source 11. And the base of the transistor 2 to be controlled is connected with a series node of the first transistor 13 and the second transistor 14, the collector of the same being connected with an exterior power source (not shown).
Next, explanation will be made on the operation of such a conventional driving circuit for a semiconductor element.
In the case, for example, where a signal from the exterior is not inputted to the light emitting element 15a of the photocoupler 15, accordingly, the light receiving element 15b does not receive a photo signal, that is, the photocoupler 15 is in the off state, the first transistor 13 is conducted as base current is supplied through the resistance 16. As a result, as a circuit is formed in the order of the first power source 11, resistance 17, collector and emitter of the first transistor 13, base and emitter of the transistor 2 to be controlled, first power source 11 to flow forward bias current to the transistor 2 to be controlled, the transistor 2 to be controlled is conducted.
On the other hand, when the photocoupler 15 is in the on state, as the second transistor is forward-biased to be conducted, a circuit is formed in the order of the second power source 12, emitter and base of the transistor 2 to be controlled, emitter and collector of the second transistor 14, resistance 18, second power source 12 to flow reverse bias current to the transistor 2 to be controlled, thereby the transistor 2 to be controlled being unconducted.
Here, the voltage of the first power source 11 is designated by V.sub.B1, resistance value of the resistance 17 R.sub.17, and voltage between the base and emitter of the transistor 2 to be controlled V.sub.BE, forward bias current I.sub.B1 is given by the following equation (1). EQU I.sub.B1 =(V.sub.B1 -V.sub.BE)/R.sub.17 ( 1)
In addition, the voltage of the second power source 12 is designated by V.sub.B2, resistance value of the resistance 18 R.sub.18, and voltage between the emitter and base V.sub.EB, reverse bias current I.sub.B2 is given by the following equation (2). EQU I.sub.B2 =(V.sub.B2 -V.sub.EB)/R.sub.18 ( 2)
By the way, in the conventional driving circuit for a semiconductor element, in the case where the output of the driving circuit 3 is short-circuited due to miswiring of the transistor 2 to be controlled, there was a problem that the first transistor 13 and the second transistor 14 are easily burnt due to the short-circuited current. Especially, at the reverse bias side, after the transistor 2 to be controlled is in the non-conductive state, large current is required to be flown in order to absorb storage carrier even for a short time (about 1 to 20 .mu.m sec.). Therefore, the resistance value R.sub.18 of the resistance 18 is required to be designed small, which makes abnormal current at the time of short-circuiting to be large, thereby burnout of the first transistor 13 and the second transistor 14 are easily caused and protection of the driving circuit 3 are not carried out satisfactorily.